1. Field of the Invention
The present invention relates to a control apparatus for a vehicular automatic transmission which is provided with a plurality of hydraulic engaging elements and which performs shifting (or speed changing) of a plurality of transmission trains (or speed stages) by supplying hydraulic oil to, and discharging it from, these hydraulic engaging elements. In this specification, the term "vehicular automatic transmission" means an automatic transmission for a vehicle such as a motor vehicle.
2. Description of Related Art
Conventionally, as a control apparatus for this kind of automatic transmission, there is known one in which a plurality of shift control valves corresponding to a plurality of hydraulic engaging elements are provided so that supply of hydraulic oil to, and discharge thereof from, each of the hydraulic engaging elements can be independently controlled by each of the shift control valves. This control apparatus, however, has the following disadvantage. Namely, when the shift control valves fail (or are poorly or wrongly operated), hydraulic engaging elements other than those for establishing a desired transmission train are also engaged, with the result that there occurs a so-called simultaneous engagement in which a plurality of transmission trains are established at the same time.
In order to solve this kind of problem, there is known an apparatus in Japanese Published Unexamined Patent Application No. 42681/1996. In this apparatus, a plurality of shift control valves are divided or grouped into two groups, i.e., a first group of shift control valves corresponding to those hydraulic engaging elements ice for establishing a predetermined transmission train, and a second group of shift control valves corresponding to the other hydraulic engaging elements. Between an oil supply passage which is connected to a hydraulic pressure source and the second group of shift control valves, there is interposed a fail-safe valve which can be switched between an open position in which the oil supply passage is connected to the second group of shift control valves and a closed position in which this connection is shut off. At the time of failure in which the hydraulic oil is supplied to all of such plurality of hydraulic engaging elements as are not to be engaged at the same time, the above-described fail-safe valve is switched to the closed position. In this manner, the oil supply passage is connected only to the above-described first group of shift control valves to thereby prevent the occurrence of simultaneous engagement.
In the above-described conventional apparatus, the object of connecting the oil supply passage to the first group of shift control valves at the time of failure is to supply hydraulic oil to the hydraulic engaging elements corresponding to the first group of shift control valves to enable the vehicle to run by establishing a predetermined-speed transmission train. There is, however, a possibility that the hydraulic oil can no longer be supplied to the corresponding hydraulic engaging elements due to the failure in the first group of shift control valves. It is therefore desirable to further improve the redundancy of the fail-safe function.
In view of the above-described points, the present invention has an object of providing a control apparatus in which, even if control malfunction occurs to the first group of shift control valves at the time of failure, the hydraulic oil can be supplied to the corresponding hydraulic engaging elements so that a predetermined-speed transmission train can be surely established.